A carbon nanotube, which is formed with carbon atoms arranged in a tubular shape having a diameter of a nanometer level, has been receiving considerable attention in recent years as a carbon-based highly functional material having advantages such as high conductivity and mechanical strength. As one method of generating the carbon nanotube, a thermal decomposition method has been devised, in which thermal decomposition of a material gas such as an alcohol-based or hydrocarbon-based gas is performed in a heating furnace using a catalyst particle having a diameter of a nanometer level to grow a carbon crystal on the catalyst particle to form the carbon nanotube. The thermal decomposition method includes a method in which a base material is made to carry the catalyst particle by application or the like, or a method in which a catalyst is suspended in a vapor phase.
Japanese Patent Laying-Open No. 60-054998 (Patent Document 1), for example, proposes a method of generating a vapor phase epitaxy carbon fiber in a suspended state by heating a mixed gas including a gas of an organotransition metal compound, a carrier gas and a gas of an organic compound to 800-1300° C.
Japanese Patent Laying-Open No. 2001-020071 (Patent Document 2) proposes a method of synthesizing a carbon nanotube including a step of forming a catalyst metal film on a substrate, a step of etching the catalyst metal film to form an isolated nano-sized catalyst metal particle, and a step of supplying a carbon source gas into a thermochemical vapor phase deposition device to grow a carbon nanotube on each of the isolated nano-sized catalyst metal particle by a thermochemical vapor phase deposition method to form a plurality of carbon nanotubes aligned vertically on the substrate, in which the step of forming the isolated nano-sized catalyst metal particle is performed by a gas etching method using at least one eching gas selected from the group consisting of an ammonia gas, a hydrogen gas and a hydride gas after thermal decomposition.
Japanese Patent Laying-Open No. 2002-255519 (Patent Document 3) proposes a method in which a hydrocarbon gas and a carrier gas are sent onto a base including a heat resistant porous carrier carrying dispersed fine catalyst particles to vapor-phase synthesize a monolayer carbon nanotube utilizing thermal decomposition of the hydrocarbon gas.
Japanese Patent Laying-Open No. 2003-292315 (Patent Document 4) proposes a method of producing a carbon nanotube on a surface of a metal by a chemical vapor phase epitaxy method with flowing a gas as a carbon source onto a heated metal, which is characterized in that a microcrystal of an oxide is generated beforehand on the surface of the metal to form minute projections and depressions on the surface of the metal.
In a conventional method as described in each of Patent Documents 1-4, however, a carbon substance such as amorphous carbon or graphite as an impurity is generated concurrently with an intended carbon nanotube during production of the carbon nanotube. In addition, generated carbon nanotubes have large variations in diameters, and it is difficult to stably produce even carbon nanotubes.
One of causes of the variations in diameters of carbon nanotubes is variations in sizes of catalyst particles. Since it is difficult to control a shape of a catalyst particle when the catalyst particle is formed by a chemical method such as heat decomposition, variations in shapes of catalyst particles themselves are generated. Aggregation of catalyst particles also causes variations in shapes. Shapes of carbon nanotubes may also vary due to variations in growth speeds of carbon crystals on the catalyst particles.
In addition, a carbon nanotube having a large fiber length cannot be easily generated using the catalyst particle.
Patent Document 1: Japanese Patent Laying-Open No. 60-054998
Patent Document 2: Japanese Patent Laying-Open No. 2001-020071
Patent Document 3: Japanese Patent Laying-Open No. 2002-255519
Patent Document 4: Japanese Patent Laying-Open No. 2003-292315